Word knowledge is critical for speaking, reading and writing, yet a substantial proportion of children with language impairment demonstrate poor word learning and consequently poor vocabulary. Because vocabulary has a causal relationship with reading comprehension, this presents a significant national health concern. To develop effective interventions we require a clear understanding of the deficits underlying poor word learning. Deficits in short-term phonological memory have been implicated as a causal factor in poor word learning; however, phonological memory measures account for a limited amount of variance associated with word learning. We hypothesize that word learning success depends on the contribution and interaction of all working memory components and that word learning variability is directly related to a child's working memory profile. To test this hypothesis we compare the fit of four theoretical working memory models and four working-memory- based word learning models in five groups of children and analyze between- and within-group differences using new working memory and word learning batteries. Specific Aims: (1) Model working memory and word learning performance in monolingual English-speaking children with typical development (TD); (2) Compare working memory and word learning performance in bilingual Spanish-English children with TD to monolingual English-speaking children with TD; (3) Compare working memory and word learning performance in monolingual English-speaking children with specific language impairment (SLI), dyslexia, and comorbid SLI/dyslexia to monolingual English-speaking children with TD; (4) Develop a unified working memory-based word learning model; (5) Determine whether working memory-based word learning subtypes exist within the typically-developing monolingual English, bilingual Spanish-English, SLI, dyslexia and SLI/dyslexia groups; (6) Finalize an efficient working memory-based word learning battery useful for research and clinical assessments. Significance: The theoretical and clinical significance accrued across these aims includes (1) data-based comparisons of four theoretically-based WM models in children, (2) the development and testing of a new working-memory-based WL model in children, (3) establishing the relationship between WM and WL in monolingual English and bilingual Spanish-English speaking children compared to children with SLI, dyslexia and SLI/dyslexia, (4) a comprehensive description of between- and within-group word learning differences in large, carefully selected typical and clinical groups and (5) the refinement of a working-memory-based word learning battery for future theoretical and clinical research. These studies address important problems in the fields of psychology, language science and education, including the need (1) for comprehensive working memory and word learning batteries for children to support valid and reliable assessment and (2) to under- stand deficits underlying poor word learning so that effective treatments can be developed. By completing this research we will advance scientific knowledge in working memory and word learning in school-age children.